Experimental evidence accumulated over the past 5 years clearly indicates that ovarian steroids regulate the number of synapses in the rat hippocampal CA1 region. When estradiol levels are high such as during proestrus and ovulation, the number of synapses is high; when estradiol levels are low such as during estrus, the number of synapses is low. Here we address three questions that are frequently raised by these phasic fluctuations in synapse number in a brain region to which cognitive functions are classically attributed. First, what neuronal signals might produce the changes in synapse number? Second, how are the hippocampal functions of memory encoding and cognitive mapping affected by fluctuating levels of ovarian steroids? Third, for mammals in general, what might be the ecological/cognitive significance of such changes? In this last section, we integrate some of the relevant human and rodent cognitive/behavioral literature and propose a hypothesis. Namely, by altering its quantitative connectivity, the female hippocampus is optimized for different cognitive/behavioral functions when the female is sexually receptive and ovarian steroid levels are high rather than when she is not receptive and steroid levels are low. The hippocampus thus shifts its optimal computational functions across the estrous/menstrual cycle.